Making Straw Based Mushroom Growing Substrate

Originally posted at Erowid.

Making Straw Based Mushroom Growing Substrate

Hot water pasteurization:
This method is especially suitable for most types of oyster mushroom, P. cubensis  Strains and Liberty Caps. Simple technology, short production cycles are the main advantages of the approach. With some experience and extra care, hot water pasteurization may also be used to prepare substrate for Shiitake, Crimini, Portobello, shaggy mane, garden giant, blewit, and the paddy straw mushroom. It is especially effective with whole or chopped straw, but may also be used for coarse wood chips, alone or in admixture with straw and small amounts of similar materials and supplements.


Details of the method:
Cut 2 kg cereal straw (wheat, barley) into 2-4 inch long pieces - enough to form a compacted, basketball-sized lump. Hay, hardwood chips or sawdust, shredded paper and peat moss may be added with appropriate changes in the technique to accommodate the altered water holding capability, nutritional content, and tendency to heating of the material. Prepare (shred, etc) the raw substrate outside or in another room from the one in which you will pasteurize and inoculate it, as straw dust is laden with weed mold spores which may contaminate and ruin your project.

Add 2 cups wheat or rice bran (or grains such as rye, sunflower seed in the hull, or millet), and 1 teaspoon Calcium carbonate (crushed limestone - not Dolomite) to the straw and place the mixture into a cloth bag (pillow case) or wire basket.

Completely submerge the bag in a vessel of hot water (65°C) for approximately 1 hr. Some heavy object will probably be necessary to keep the material completely submerged - straw which is not submerged will not be pasteurized and carry contaminants over to the next stage.

Cleanliness is essential. While you are waiting for the completion of the hot water pasteurization, wash down work areas with soap and water then wipe down with disinfectant (household bleach diluted 1:10 with water is effective). Airborne dust and mold spores on your hands, clothes, and on the counter on which you will work will lead to contamination problems.

Carefully take the container of straw out of the hot water and place it on a clean, disinfected surface to drain and cool to room temperature. This will take approximately 1-2 hours. When you are ready to proceed with the next step, wash your hands and arms well with soap and water, and wipe the counter-top again with disinfectant. It is not essential, but if rubber gloves are available, disinfect them and wear them.

Empty the cooled straw onto a clean, disinfected working surface. Place your bag or jar of spawn within easy reach. Take a large handful of the straw and shake/sprinkle it into the bottom of a large (approx. 18x24"), clear poly bag. Next take a small handful of spawn and sprinkle it on top of the straw. Spread another layer of straw and again sprinkle spawn over it. Repeat this process until the bag is full of intermixed layers of straw and spawn. Pack the mix down well as you go (by pressing on the outside of the bag as much as possible - to minimize contact with your unavoidably dirty hands). Add some more straw and spawn to the bag, if appropriate, until it is full of compacted straw - the tighter the better. Then tie the bag securely at the top.

Puncture the bag 30 - 50 times with a clean nail, knife or fork (wash it then dip in bleach).
Place the bag of spawned straw in an environment where it will be protected from insects, dust and drying out, and maintain it at a temperature of 15-20°C., not too warm initially or excessive biological heating may occur. A large, new, clean garage bag makes a nice protective tent (for a single straw block), but only fold the top of the outer bag over loosely, so as to allow for necessary ventilation. When many blocks are incubated in 1 room, air movement within the room and ventilation of the room is essential.

In about 2 weeks, when the straw is all white with the mycelium, take the small bag out of the large one and make about 10 - 1/2" slashes in the bag, both around the substrate block and in the head space, then place it in a humid, lighted fruiting environment. - Pleurotus ostreatus strains will often require a slight cold shock, down to approx. 12-15°C for 2 or 3 days to initiate fruiting. After initiation, the best quality mushrooms will be produced at approx. 13-18°C.

No cold shock is necessary for P. flabellatus, P. sajor-caju, P. pulmonarius, or P. cornucopia, and these species should be kept at approx. 15-20°C for production of the best quality mushrooms (20°C for P. flabellatus).

If mushrooms start to develop on top of the substrate, open the top of the bag. Open or cut the bag away as necessary to allow the mushrooms to develop without restriction.

Once the individual mushrooms reach about 1/2" diameter, they may be watered to promote rapid growth. But they must not remain wet for more than about an hour at a time, or they may rot. Adjust the environmental humidity to allow them dry out slowly if you water them, and do not routinely water more than once or twice per 24 hr day.

Typically, mushroom growing environments should have humidity about 80-90 They should not exceed 90except possibly for a day or 2 during "pinning." Several alternatives to the hot water immersion method that work well for oyster mushrooms on straw substrate include:

Lime water treatment:
Fill a clean, 45 gallon drum with fresh water and add 4 cups of Calcium hydroxide (hydrated lime) or Calcium oxide (quicklime) with stirring. Prepare straw as for hot water pasteurization above, then completely immerse in the lime water solution for about 2 hours. The water need not be hot, but very cold water does not seem to work well. Remove the bags to a clean place to drain, then spawn as for hot water pasteurized straw. This method is very economical as lime is cheap, but it is mild, and only good quality, clean straw should be used. Lime water will not kill grains, and sprouting grain may promote certain microbial pests. One batch of lime water will treat about 20 Kg. of straw.

Hypochlorite treatment:
Add 4 cups of household bleach solution to 45 gals. water to prepare a disinfectant solution that works about as well as lime water and is very economical. Immersion time and limitations are as for lime water. All the straw must be immersed at once. Serial treatment of a number of batches does not work well as the hypochlorite becomes depleted.

Formaldehyde and maneb treatment:
A recent article Mush.J.Tropics 7:67-75 (1987) describes an effective method for sanitizing wheat straw by immersion in dilute formaldehyde (500 ppm)/Maneb (75 ppm) solution. To prepare the solution, add 1 cup of 37ormalin solution and 1/4 tsp. Maneb powder with stirring to 45 gals water. Immerse chopped straw in the above solution for 18 hours, drain and spawn as above. Zineb or Benomyl might substitute for Maneb in the same application.

Cold water immersion:
Fill a tub with clean, cold water. Prepare straw as for hot water, and completely immerse in the water for 5-7 days. The water should have a large surface area and have some opportunity to circulate by convection, to avoid anaerobic conditions. Use good quality straw. Supplementation prior to immersion is not recommended. At the end of the immersion period, remove the straw from the water, drain and spawn as above.